I see this question has been out here unanswered for a couple months. It's a tough one to answer so I'm not surprised. While not knowing your business or niche well, here are a couple thoughts. Perhaps you've pursued these ideas already, but I thought I'd toss them out.

You mentioned PPC wasn't effective. Was that only Google and/or Bing PPC? If you haven't tried targeting that space via Facebook Ads, that might be worth a try. The targeting is excellent and the clicks are still affordable. LinkedIn Ads have less targeting options and are more expensive but depending on the niche, it could be an option - especially in B2B. There are some creative ways to broaden the audiences on these mediums by highlighting conferences, trade mags, and industry websites as interests of the audience.

Do you have a list of targeted accounts in the niche? Account Based Marketing (ABM) is great way to micro target your ideal customer profile. It's not easy but can help you build awareness at the very least.

Are there some influencers or known experts in the space you could reach out to directly with your content? Have someone else in the space share your content with their following could help you reach more people.

I know directly contacting someone on LinkedIn can be intrusive but if you're approaching with content they'd find valuable, the outreach could be well-received. Linking out to experts via a blog post or Twitter post can also get attention.

Engaging influencers on LinkedIn and Twitter by first sharing their content can put you on their radar making the outreach a little warmer when you do directly contact them.

Are there smaller conferences or meet-ups in the niche you could sponsor or attend as a sponsor? Oftentimes, you'll get the attendee list as a sponsor and can follow-up with an email campaign. If the email campaign was in the form of a drip campaign, you could warm them up with your content and then follow-up with a compelling offer.

With all of these above, make sure that your remarketing tags are set on your site. If you do get the traffic, make sure they're seeing you again after they leave on Facebook, LinkedIn, and other niche sites via Google PPC.

Hope that offers a couple ideas. Happy to brainstorm further if you'd like.